I tested this on a recent Penzance to Cardiff run, specifically to see what they said.
We had a Castle HST from Penzance- no 1st class was available at the time of booking, or on the day.
At Plymouth, a 5-car IET was subbed.
We asked the TM if 1st was going to be declassified. He did not know, but asked.
Eventually, when we did the transfer, we set in 1st and continued the discussion.
We were told that we could pay to upgrade to 1st (even though the 1st fare was not available).
The TM said that this was because GWR charged 1st class fare from Plymouth (though not to Cardiff).
The price we were quoted was comedy- possibly more than £70 each- this was on a Thurs evening service, so it wasn't busy.
Later on, people came along and just sat in it, but weren't challenged (but they looked like they might kick off!!).
Looks like they don't know what they are doing, or are trying to skim money off customers.
I also got charged a 1st class upgrade that I later discovered was double the actual walk on 1st class fare.
I think GWR need scrutinising, as there seems to be a evidence that passengers may be being ripped off.
On the other hand, you can pick up a 1st class advance single Cardiff to Paddington for £57, which seems reasonable as you can use the lounges and there is a proper at seat service.